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GEOSYNTHETICS FOR FILTERS AND DRAINS

Faculty: Samirsinh .P.ParmarDepartment of Civil EngineeringFaculty of Technology, Dharmasinh Desai University,Nadiad, Gujarat, INDIAMail Add: samirddu@gmail.com

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FOLLOWING FUNCTIONS MUST BE FULFILLED

Maintain adequate permeability Prevent significant washout of soil particles Avoid accumulation of soil particles within

the geosynthetic structure Survive the installation stresses and any

other long-term mechanical, biological or chemical degradation impacts for the lifetime of the structure

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WHILE ESTABLISHING GEOSYNTHETIC FILTER CRITERIA FOR DRAINAGE APPLICATIONS, THE FOLLOWING BASIC FILTRATION CONCEPTS ARE KEPT IN MIND:

If the largest pore size in the geotextile filter is smaller than the larger particles of soil, then the filter will retain the soil.

If the smaller openings in the geotextile are sufficiently large enough to allow smaller particles of soil to pass through the filter, then the geotextile will not blind or clog.

A large number of openings should be present in the geotextile so that proper flow can be maintained even if some of the openings later become clogged.

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THE PERMEABILITY CRITERIA OF GEOTEXTILE FILTERS, COMMONLY SUGGESTED, ARE IN THE FOLLOWING FORM:

where kn is the coefficient of the cross-plane

permeability of the geotextile; ks is the coefficient of permeability of the

protected soil; and A is a dimensionless factor varying over a wide

range, say 0.1 to 100.

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FOR GEOTEXTILE FILTERS, TYPICAL PERMEABILITY CRITERIA FOR SOME SPECIFIC APPLICATIONS

For a standard drainage trench:

For a typical dam-toe drain:

For dam clay cores:

It must be noted that the critical applications may require the design of even higher kn/ks ratio values, due to the high gradient that can occur in the filter vicinities.

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FEDERAL HIGHWAY ADMINISTRATION (FHWA) ALSO ESTABLISHED THE FOLLOWING PERMITTIVITY REQUIREMENTS FOR SUBSURFACE DRAINAGE APPLICATIONS (HOLTZ ET AL., 1997):

For < 15% passing 75 μm:

For 15–50% passing 75 μm:

For > 50% passing 75 μm:

•Retention criteria govern the upper (piping) filtration limit for filters and ensure that the soil to be protected is not continually piped through the geotextile filter and into the drainage medium.

•Failure to adopt appropriate retention criteria for filter design can have costly and potential catastrophic consequences.

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THE RETENTION CRITERIA OF GEOTEXTILE FILTER COMMONLY SUGGESTED ARE IN THE FOLLOWING FORM:

Where: • Of is a certain characteristic opening size of geotextile filter; • Ds is a certain characteristic particle diameter of the soil to be protected, it indicates particle diameter, such that s%, by weight, of the soil particles are smaller than •Ds and B is a dimensionless factor varying over a certain range.The magnitude of B depends on a number of

factors, including soil types, hydraulic gradient, allowable amount of soil to be initially piped, the test method to determine Of and Ds, and state of loading (confined and unconfined)

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(a) General procedure for determining the piping limit

(b) determination of piping limit for Hong Kong Completely Decomposed Granite (CDG) soils

The piping limit conventionally is established as the maximum stable ratio of O95/D85 below which soil is not continually piped through the geotextilefilter.

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To overcome the time element associated with permeameter testing of individual soil types, standard retention criteria have been developed over a wide range of groups in the past.

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THE SIMPLIFIED DESIGN PROCEDURES FOR A GEOTEXTILE FILTER FOR STABLE SOILS IN SUBSURFACE DRAINAGE SYSTEMS

Step 1: Evaluate the soil to be filtered (the retained soil). As a minimum, this should include:

● visual classification ● consistency limits ● particle size distribution analysis. Step 2: Determine the minimum survivability requirements. Step 3: Determine the minimum permeability using the

permeability criterion. Step 4: Determine the maximum opening size using the

retention criterion as well as clogging criterion. Step 5: Select the geotextile in accordance with Steps 2, 3, and

4. Step 6: Perform a filtration test to meet the requirements of

retention and anti-clogging criterion, if the application is critical.

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IF GEOCOMPOSITE DRAINS ARE BEING USED FOR DRAINAGE APPLICATIONS, THEN THEIR DESIGN MUST SATISFY THE FOLLOWING CRITERIA (CORBET, 1992):

1. The core must resist the applied loads (normal and shear) without collapsing.

2. Under sustained load the core must not reduce significantly in thickness (compression creep).

3. The core must allow the expected water flow to reach the discharge point without the buildup of water pressure in the core.

4. The core must support the geotextile filter.

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GEOTEXTILE FOR DRAIN AND BEHIND RETAINING WALL

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COMPARISON OF GEOTEXTILE WITH GRANULAR FILL

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GUIDELINES FOR EVALUATING CRITICAL / SEVERE NATURE (ADOPTED FROM GEOSYNTHETICS ENGINEERING – AUGUST 2008)

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For less critical applications and less severe conditions,

For critical applications and severe conditions,

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PERMITTIVITY

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TRANSMISSIVITY

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GT FILTER FIELD INSTALLATION PROBLEMS

Lack of intimate contact (GT-to-soil) generally occurs with fabric vertical or

inclined can also occur by uplifted horizontal fabric wrinkles and folds generally not problematic many reported problems from the field

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FACTORS TO BE CONSIDERED FOR GT FILTER DESIGN: Define Soil Particle-Size Distribution Define Soil Atterberg Limits Determine the Maximum Allowable

Geotextile Opening Size (O95) Define the Soil Hydraulic Conductivity (ks) Define the Hydraulic Gradient for the

Application (is) Determine the Minimum Allowable Geotextile

Permeability (kg)

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IN PLANE PERMEABILITY FILTER

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TYPES OF GEOTEXTILES USED AS FILTER FABRICS

Woven Slit (Spilt) Film

Woven Monofilament

Nonwoven Heat Bonded

Nonwoven Needle Punched

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SILT FENCES BY GEOTEXTILES

GT used as silt fences!

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GT FILTER IN EARTH DAM

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QUESTIONS ?